1. Field of the Invention
This invention relates to a hot wire type of air flow meter, and more particularly to a hot wire type of air flow meter as used in an air intake system of an automobile for detecting and controlling the quantity of intake air.
2. Background of the Invention
Hitherto, as disclosed in Japanese Patent Unexamined Publication No. 50-50520, Japanese Patent Unexamined Publication No. 50-146369 and Japanese Patent Unexamined Publication No. 55-69021, there is a hot wire type of air flow meter for an internal combustion engine so constituted that a straight-shaped bypass (branch pipe) is disposed in the central portion of a main pipe and hot wire elements are disposed therein. However, in the structure disclosed in Japanese Patent Unexamined Publication No. 50-50520, such hot-wire element is not protected from reverse heat due to backfire caused from incorrect engine ignition timing. To this end, a structure for protecting the hot wire element against backfire has been disclosed, for example, in Japanese Patent Unexamined Publication No. 55-69021. However, a hot wire element generally has characteristics such that the output therefrom is reduced although the average flow velocity increases in a case where the same is disposed in a flow having a large pulsation due to the nonlinear heat transfer coefficient thereof. Therefore, any of the above-described structure cannot correctly detect the flow rate of a pulsation flow.
As disclosed in Japanese Utility Model Unexamined Publication No. 56-135127, in a bypass in which a hot wire element is disposed in the main pipe, in order to perform protection against the above-described backfire or correctly detect the flow rate of the pulsation flow, the fluid resistance at the bypass in the lower stream to the hot wire element is enlarged, additionally, the outlet and inlet ports of the bypass are formed in parallel or substantially in parallel to the main stream. That is, the dynamic pressure acting at the outlet and inlet ports of the bypass is reduced even if a reverse flow is generated and the flow reaching the hot wire element also is damped so that backfire resistance is improved. Since the outlet port of the bypass opens directly and substantially in parallel to the main stream, the flow in the bypass is slightly changed due to the static pressure change caused from mixture of flows at this portion. It leads to generation of noise in the output from the hot wire element. Although high frequency noise can be reduced by a filter disposed in the circuit, the abovedescribed type of noise causes a system control problem when, for example, the engine is operated at a low speed. Furthermore, from the viewpoint of hardware, the structure produces production (cost and weight) and reliability (the number of parts) problems since the axial length is too long and the component parts for the bypass are difficult to mount.
On the other hand, structures have been disclosed, for example, in Japanese Patent Unexamined Publication No. 56-76012, in which a bypass in which a hot wire element is disposed is formed outside the main stream in order to prevent the above-described type of backfire and stabilize the outputs.
However, problems arise in these structures, as pointed out in Japanese Patent Unexamined Publication No. 56-76012, that flow rate detection error in large due to the thermal conditions, such as thermal conduction from the engine, heating of the hot wire element, and in the case of an automobile, engine heat and rise in temperature in the engine compartment due to solar radiation. That is, since the bypass portion provided with the hot wire element has a large thermal capacity and is formed with a relatively tight width in the inner portion of a body wall, there is not a large area for conducting the heat of the intake air. Furthermore, the bypass is formed to perform a good thermal conduction of the air flow passing therein. Therefore, the temperature of the air flow in the bypass is affected by the temperature of the passage wall of the bypass, causing a large temperature difference from that of the main flow. It leads to enlargement of the error in measurement of the intake air flow rate.
Some of the disclosures relate to a structure which cannot withstand strong engine backfire and are incapable of correctly detecting the average flow rate of the pulsation flow, so that such devices cannot be put to practical use. Some of them are incapable of correctly measuring the flow rate under certain thermal conditions and are also incapable of being sufficiently controlled to have the engine operated at the most suitable air-fuel ratio due to increase in the noise in the output. Therefore, it interrupts cleaning of the exhaust gas from the engine, improvement in fuel consumption and operability. On the other hand, some of them are insufficient in reduction of the axial length of the main flow meter body, that is, they are insufficient in reduction of the length of the intake pipe, weight of the body and manufacturing cost. Therefore, problems arise that the pressure loss in the intake pipe increases, and the weight of the system including the engine becomes heavier, preventing improvement in engine fuel consumption and reduction of the engine room space.